Studies of the domain structure of rat brain tubulin has revealed the following domains: 1. Larger N-terminal and smaller C-terminal domain in both alpha and beta tubulin. These remain attached in the dimer under native conditions despite single bond cleavage in each monomer and microtubules can be formed from the split material. Intradimer contacts are beta-C terminal domain to alpha-N terminal domain and the interdimer contacts in the microtubule are alpha-C terminal to beta-N terminal. 2. The equilibrium association between the alpha and beta monomers has been studied by two indirect means: a) exposure for proteolysis and b) binding of the fluorescent dye, mile red, at the alpha/beta interface, and directly by equilibrium certifugation with a new short column method. Dissociation constants vary from 0.2 - 1.0 muM. 3. Thermodynamic analysis of dimerization by equilibrium ultracentrifugation reveals that monomer association is entropy-driven, is hydrophobic, and is sensitive to small ligands such as cholchine and GTP. 4. The colchicine binding site has been localized to the beta-monomer by direct photolabeling. It appears to be near enough the alpha-subunit so that the latter may contribute to binding.